It is known to protect critical loads supplied with AC power, by providing at least one power processor for, for instance, filtering harmonics in the power supply voltage, and circuit breakers to protect the electric circuitry from overcurrents in the case of a short-circuit at the critical load. However, such power processors have a weak overcurrent shortcircuit capacity. For this reason, the power supply system of the closest prior art, as represented in FIG. 1, comprises, besides a power processor 1 (in this example, an AC/DC/AC converter) and a first bidirectional switch 16 connected in series between a power input 2 with a nominal input voltage VNin and an AC power output 3 having a nominal output voltage VNout and a nominal output current INout, a power backup circuit 6 connected in parallel to said power processor 1 between said power input 2 and AC power output 3 and possibly comprising a second bidirectional switch 7 and control means (not illustrated) for opening said first bidirectional switch 16 and closing said second bidirectional switch 7 after detecting a short circuit condition downstream of the AC power output, by, for example, detecting if the voltage at the AC power output 3 of said power processor 1 drops below a threshold set at less than 80% of said nominal output voltage VNout. The power supply safety circuit 6 will therefore bypass said power processor 1 in the case of a shortcircuit at a critical load 5 to which it is connected through an individual circuit breaker 4. A similar power supply system and method was disclosed in International Patent Application WO 2004/102785.
This arrangement has the drawback that a higher-rated circuit breaker 8 upstream of the power processor 1 can open at the same time as the individual circuit breaker 4 downstream of the power processor 1. Therefore, if the power processor 1 is connected to a plurality of loads 5, as illustrated, a shortcircuit in one of them will lead to the power supply to all of them being interrupted. With critical loads, this is of course undesirable, but providing each individual critical load with its own power processor and corresponding bidirectional switch would be complicated and costly.